Histidine-containing protein (HPr) is a phosphotransferase in the bacterial phosphoenolpyruvate-dependent phosphotransferase system. HPr provides a rich model system in which to study a number of important aspects of protein structure/function, including protein phosphorylation and protein-protein interactions. HPr can be phosphorylated at two different sites: phosphorylation of a histidine residue occurs as an intermediate in the phosphotransfer reaction and phosphorylation of a serine residue modulates the activity of the protein. The structural and dynamic ramifications of protein phosphorylation will be studied using HPr. As well, the interaction between HPr and its phosphoryl-acceptor protein, Enzyme IIA will be studied. To gain detailed information about these important aspects, solution structures of native HPr, its two phosphorylated forms, and mutant forms of the protein with altered functional properties. Continuing developments in NMR spectroscopy make this a powerful technique with which to probe the details of both structural and dynamical properties of proteins in solution.